STBMAUINK TfNNKI.IM; 



255 



Glasgow Port tunnel, are of the same general design and con- 

 struction. To represent sliields of the second class, the St. 

 ('lair River and Blackball shields are shown in Figs. 128 and 

 I:?! 1 . The shields for the Mersey River, the Hudson Ki\ei, 

 and the East Iliver tunnels also l>elong to this class. To 

 represent shields of the third class, the elliptical and senii- 



ff 



Otlo.lt of Cmtmq under CnH 

 of Mrdm. 



Longitudinal Section t-0. 



FKI. 132. K-xif Shield for Ibwton Subway. 



elliptical shields of the Cliehy tunnel work in Paris are shown 

 liy l-'i^s. 1UO ami 1:51. The semi-circular shield of the Boston 

 Snhway is illustrated hy Fig. 1: '.'_'. 



SHIELD CONSTRUCTION. 



General Form. Tunnel shields are usually cylindrical or 

 s'-ini-cylindriral in cross-section. The cylinder may IK) circular, 

 elliptical, or oval in section. Far the greater nnnd>cr of shields 

 used in the past have IH-CII circular cylinders; hut in one part 

 of the srurr tunnel of Cliehy, in Paris, an elliptical shield 



